Under a scenario of cognitive radio (CR), in order to allow a secondary system (SS) without a licensed spectrum to have an opportunity to access a licensed spectrum of a primary system (PS), the primary system needs to have a capability of discovering a spectrum usage. A user in the primary system (which is referred to as a secondary user (SU)) senses the spectrum being used by the primary system, judges whether there is a primary user (PU) (i.e., a user in the primary system) signal, and decides how to use the spectrum of the primary system without interfering the PU.